It is a commonly known fact that when one places two conducting electrodes connected to a voltmeter, one on the scalp and the other on an electrically neutral area, such as the mastoids behind the ears, a quantifiable voltage can be observed. This voltage signal and its change with respect to time is the basis of electroencephalography, or EEG. The signal measured on the scalp is actually a summation of individual postsynaptic potentials occurring within the brain. Since both the neural tissue and the skull act as a low pass filter, it is unlikely that the high frequency transients of action potentials would make it up to the scalp, and since postsynaptic potentials generally have lower frequency transients associated with them, it is widely believed that the observed EEG signal originates from them.
The EEG recording is characterized by amplitude, frequency and their change over time. The frequency component of the EEG can be utilized to infer the level of an individual's neural activity. The frequencies are broken down into ranges which describe how alert and conscious a person is at any given time. The delta frequency (1-4 Hz) is associated with deep sleep. The theta frequency (5-7 Hz) is associated with drowsiness, and delta activity is also common. The alpha frequency (8-13 Hz) is associated with relaxed wakefulness, where not much brain resources are devoted to any one thing. The beta frequency (12-20 Hz, or 30 Hz) and the gamma frequency (36-44 Hz) are associated with alert attentiveness.